The pituitary, an endocrine gland primarily regulated by the hypothalamus, secretes multiplehormones and regulates the release of several other hormones from multiple glands within the body. Pituitary development is conserved across vertebrate species and involves a complex temporal and spatial balance of multiple transcription and signaling factor gradients, which control cell commitment, differentiation, and proliferation. Proper pituitary development is critical to survival, since several essential physiological processes are regulated by the pituitary during embryogenesis and throughout life, including metabolism, milk production, stress, reproduction, and growth. Glucocorticoids (GCs) produced by the adrenal glands during embryogenesis play an important role in the differentiation of somatotrophs, the endocrine cell type within the pituitary that produces growth hormone (GH), as well as GH mRNA expression in both avian and mammalian species. In chickens, somatotrophs differentiate between e12 and e16, while mouse somatotrophs differentiate between e15.5 and e17.5. The establishment of the

hypothalamic-pituitary-adrenal (HPA) axis and a rise in circulating levels of corticosterone(CORT), the primary GC in rodents and chickens, coincides with somatotroph differentiation and GH mRNA expression in both rodents and chickens. Furthermore, exogenous synthetic GCs such as dexamethasone have been shown to prematurely differentiate somatotrophs in rats and chickens in vitro and in vivo. GCs generate physiological reactions by binding to target cells that express the glucocorticoid receptor (GR). GR, in turn, acts as a transcription factor and regulates the expression of several genes. Despite numerous studies on GH mRNA regulation and somatotroph differentiation by GCs, the mechanism is not completely understood. To better understand the role GR plays in GH regulation by GCs in a mammalian model, mouse embryos with pituitary-targeted GR knockout were generated utilizing the Cre-LoxP Recombinase system under control of the pituitary-specific αGSU promoter. GH mRNA was significantly decreased in GR(-/-) embryos, while GR(+/-)

embryos expressed intermediate levels of GH mRNA in

comparison to wild-type GR(+/+) embryos. Significant differences in expression of other pituitaryhormones in GR(-/-)

embryos were not observed, indicating that GR must not play an essentialrole in regulating the expression of any other pituitary hormone gene. Furthermore, all GR(-/-) embryos died at birth, or soon after. To our knowledge, this is the first study to report homozygous GR knockout in the pituitary suppresses embryonic GH expression and results in a neonatal lethal phenotype.